hi, have got a fault with my lupo, 1.4 8v automatic, engine management light has come on, i have read the fault codes stored in ecu and this is what i've got:

17598-linear lambda probe, reference voltage implausible sigal- no sympton
17604- lambda sensor 1 bank 1 heating circuit electrical fault

after getting some advice from an independent garage, i have replaced the post cat lambda sensor, which has resulted in the above fault codes being able to be erased but after driving car for a few minute enine light came back on.i checked the codes again and it has come up with a slightly different code

17589- linear lambda probe, reference voltage open circuit

i am puzzled as to why it now says this,as sure it should have cured the fault, have i got another fault or do i need to change the other lambda sensor( another £100 at least)
any help would be greatly appreciated as getting to be a pain!!!